An organic light emitting phenomenon is an example of a conversion of current into visible rays by an internal process of a specific organic molecule. The organic light emitting phenomenon is based on the following principle. When an organic material layer is interposed between an anode and a cathode, if voltage is applied between two electrodes, electrons and holes are injected from the cathode and the anode to the organic material layer. The electrons and the holes injected into the organic material layer are recombined to form an exciton, and the exciton is reduced to a bottom state to emit light. An organic light emitting diode using the principle may be generally constituted by a cathode, an anode, and an organic material layer interposed therebetween, for example, an organic material layer comprising a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer.
The material used in the organic light emitting diode is mostly a pure organic material or a complex compound where an organic material and metal form a complex, and may be classified into a hole injection material, a hole transport material, a light emitting material, an electron transport material, an electron injection material and the like according to the purpose. Herein, an organic material having a p-type property, that is, an organic material which is easily oxidized and electrochemically stable while the organic material is oxidized, is mainly used as the hole injection material or the hole transport material. Meanwhile, an organic material having an n-type property, that is, an organic material which is easily reduced and electrochemically stable while the organic material is reduced, is mainly used as the electron injection material or the electron transport material. A material having both p-type and n-type properties, that is, a material that is stable when the material is oxidized and reduced, is preferable as the light emitting layer material, and a material having high light emitting efficiency for converting the exciton into light when the exciton is formed is preferable.
In addition, it is preferable that the material used in the organic light emitting diode further have the following properties.
First, it is preferable that the material used in the organic light emitting diode have excellent thermal stability. This is because joule heat is generated by the movement of electric charges in the organic light emitting diode. Recently, NPB, which has mostly been used as the hole transport layer material, has a glass transition temperature of 100° C. or lower, and thus there is a problem in that it is difficult to use NPB in an organic light emitting diode requiring a high current.
Second, holes or electrons injected into the organic light emitting diode should be smoothly transported to a light emitting layer, and the injected holes and electrons should not be released out of the light emitting layer in order to obtain an organic light emitting diode that is capable of being driven at low voltage and has high efficiency. To this end, a material used in the organic light emitting diode should have an appropriate band gap and HOMO or LUMO energy level. In the case of PEDOT:PSS currently used as a hole transport material in an organic light emitting diode manufactured by a solution coating method, since a LUMO energy level thereof is lower than that of an organic material used as a light emitting layer material, it is difficult to manufacture an organic light emitting diode having high efficiency and a long life span.
In addition, the material used in the organic light emitting diode should have excellent chemical stability, electric charge mobility, and interfacial characteristic with an electrode or an adjacent layer. That is, the material used in the organic light emitting diode should be little deformed by moisture or oxygen. Further, appropriate hole or electron mobility should be ensured so as to balance densities of the holes and of the electrons in the light emitting layer of the organic light emitting diode, thus maximizing formation of excitons. In addition, an interface with an electrode comprising metal or metal oxides should be favorable for stability of the diode.
In order to sufficiently exhibit excellent characteristics of the aforementioned organic light emitting diode, a material forming the organic material layer in the diode, for example, the hole injection material, the hole transport material, the light emitting material, the electron transport material, the electron injection material, and the like should be supported by stable and efficient materials in advance, but development of a stable and efficient organic material layer material for organic light emitting diodes has not yet been sufficiently made, such that there is still a demand for developing a new material.